In conventional mass spectrometry techniques, such as "MS/MS" and "CI" methods, ions having mass-to-charge ratio within a selected range are stored in a quadrupole ion trap. The stored ions are then allowed (or induced) to dissociate or react, and the resulting product ions are then ejected from the trap for detection.
For example, U.S. Pat. No. 4,736,101, issued Apr. 5, 1988, to Syka, et al., discloses an MS/MS method in which ions (having a mass-to-charge ratio within a predetermined range) are trapped within a threedimensional quadrupole trapping field. The trapping field is then scanned to eject unwanted trapped ions (ions other than parent ions having a desired mass-to-charge ratio) sequentially from the trap. The trapping field is then changed again to become capable of storing daughter ions of interest. The trapped parent ions are then induced to dissociate to produce daughter ions, and the daughter ions are ejected sequentially from the trap for detection.
In order to eject unwanted trapped ions from the trap prior to parent ion dissociation, U.S. Pat. No. 4,736,101 teaches that the trapping field should be scanned by sweeping the amplitude of the fundamental voltage which defines the trapping field.
U.S. Pat. No. 4,736,101 also teaches that a supplemental AC field can be applied to the trap during the period in which the parent ions undergo dissociation, in order to promote the dissociation process (see column 5, lines 43-62), or to eject a particular ion from the trap so that the ejected ion will not be detected during subsequent ejection and detection of sample ions (see column 4, line 60, through column 5, line 6).
U.S. Pat. No. 4,736,101 also suggests (at column 5, lines 7-12) that a supplemental AC field could be applied to the trap during an initial ionization period, to eject a particular ion (especially an ion that would otherwise be present in large quantities) that would otherwise interfere with the study of other (less common) ions of interest.
European Patent Application 362,432 (published Apr. 11, 1990) discloses (for example, at column 3, line 56 through column 4, line 3) that a broad frequency band signal ("broadband signal") can be applied to the end electrodes of a quadrupole ion trap to simultaneously resonate all unwanted ions out of the trap (through the end electrodes) during a sample ion storage step. EPA 362,432 teaches that the broadband signal can be applied to eliminate unwanted primary ions as a preliminary step to a chemical ionization operation, and that the amplitude of the broadband signal should be in the range from about 0.1 volts to 100 volts.
In another class of conventional mass spectrometry techniques (such as the technique described in U.S. Pat. No. 3,334,225, issued Aug. 1, 1967, to Langmuir), ions injected into a quadrupole mass filter translate (at least initially) along the filter's axis. The mass filter has elongated electrodes that are oriented parallel to the filter's axis, and a quadrupole electric field is established in the region between the electrodes by applying a voltage (having an RF component, and optionally also a DC component) across at least one pair of the electrodes. The electric field allows only selected ions (having mass-to-charge ratio within a selected range) to translate axially through the filter (to the filter's outlet end) and may reject undesired ions by ejecting them radially away from the filter axis. The selected ions can be detected by a detector positioned along the filter axis beyond the outlet end.
It is conventional to apply a notch filtered broadband voltage signal to the electrodes of a quadrupole mass filter for the purpose of eliminating a range of ions having mass-to-charge ratio outside a desired range (the range associated with the voltage signal's "notch"). Such a notch filtered broadband voltage signal will be denoted herein as a "filtered noise" signal.
However, filtered noise signals have not been applied to a quadrupole mass filter in a manner facilitating mass analysis (i.e., the selective transmission of a consecutive or non-consecutive mass sequence of ions through the filter). Thus, for example, U.S. Pat. No. 3,334,225 teaches application of a single, static filtered noise signal to a quadrupole mass filter, to pass ions having mass-to-charge ratio in a single range. Until the present invention, it was not known how to perform mass analysis with dynamic mass resolution (to maintain substantially constant mass separation over a wide mass range) by applying a time-varying filtered noise signal to a quadrupole mass filter.
Conventional apparatus (such as the circuitry described in U.S. Pat. No. 3,334,225) for applying filtered noise signals to quadrupole mass filters would be incapable of applying filtered noise signals in a rapid sequence (and thus incapable of applying, in effect, a notch having time-varying width and center), or incapable of applying such a filtered noise signal sequence in a manner providing sufficient mass resolution to facilitate mass analysis over typical mass ranges of interest. The latter problem occurs in operation of conventional quadrupole mass filters due to the inverse relation between ion mass, m, and the conventional quadrupole field stability parameter q: EQU q=2eV/[mr.sup.2 w.sup.2 ],
where V is the amplitude of a sinusoidal RF voltage applied to the mass filter, "r" represents radial distance from the central longitudinal axis of the filter, "e" is the charge of an electron, and "w" is the angular frequency of the applied sinusoidal RF voltage. Because of the inverse relationship between mass and the parameter q, if one simply ramps the range of ion mass-to-charge ratios) using a conventional quadrupole mass filter, it is not possible to achieve substantially constant mass separation during the mass analysis operation.